This application claims the priority benefit of European patent application EP 99 121 608.6 filed on October 29, 1999.
The present invention relates to a process and apparatus for carrying out a multi-phase reaction using the counter current principle of a liquid and a gaseous phase.
In chemistry, one is frequently confronted with the problem of carrying out multi-phase reactions in the form of separation processes, in particular chemical reactions in multi-phase systems. Decisive in these reactions is the phase transfer velocity of the components of the respective phases to be separated or reacted. The velocity of the reaction, as is generally known, can substantially be increased by enlarging the contact surfaces of the respective phases. Such enlargement can be achieved by intensively mixing the phases. Moreover, it is known that multi-phase reactions proceed particularly efficiently and rapidly, especially, if the process is continuously carried out in the counter current flow. However, when using the counter current principle, serious problems are encountered as will later be described so that the co-current principle is employed as well.
A special problem of multi-phase reaction is constituted by gas-liquid or gas-solid-phase reactions, particularly in the cases where gaseous components in a carrier gas are reacted with emulsified or suspended components in liquid phases. In view of the extremely great differences of the mass density of gases, on one side, and liquids, or solids, on the other side (at normal pressure, a ratio of about 1:1000 and more is found), giant gas volumes have to be brought into contact with relatively small, sometimes solid-containing, liquid volumes.
In gas-liquid multi-phase reactions, liquid mixtures are often brought into the reaction. The flow behavior of such mixtures, particularly when present in the form of emulsions, is extremely complicated. Surface phenomena, for instance, might lead to partial breaking of the emulsion. Different flow velocity reducing properties, such as, for example, differences of the viscosities of the liquids and/or differences in the adhesion of the liquids on the reactor surfaces might cause disturbances when carrying out the gas-liquid multi-phase reactions.
The complexity of gas-liquid multi-phase reactions and of the physical and chemical behavior of the respective components, particularly in the case of a heterogeneously combined liquid phase, is so high that forecasting the flow behavior thereof in reactors is practically impossible. It should for instance be noted that even in the case of two immiscible liquid phases, two types of emulsions might occur, namely phase 1 in phase 2 and phase 2 in phase 1. Flow behavior, viscosity, adhesion and other physical and chemical properties of the two emulsion types can show considerable differences so that in the case of chemical reaction, the flow behavior of the liquid phase in the reactor might change.
Known methods for increasing the efficiency and the velocity of reactions with or within phase mixtures are often employed. One method includes spraying the liquids (solutions, emulsions and suspensions) into the reaction gas. Another method (preferred in the present invention) includes carrying out of the reactions in tube reactors having built-in packings for enlarging the reaction surfaces.
When carrying out gas-liquid multi-phase reactions in tube reactors according to the counter-current principle, a running-down liquid in the form of a free falling liquid film, the so-called falling film, and an ascending gas phase flowing upwardly constitute the common practice. In that kind of reactor, the gas phase of both non-packed and packed reactors always constitutes a continuous phase and moves in good approximation in the form of a plug flow through the reactor. A significant fundamental problem is encountered in such reactors, particularly in the case of high flow velocities of one or of both phases, and thus, particularly, also in the case of a very large difference in volume flow rate of gas phase and liquid phase.
In the case of ozonolysis of unsaturated fatty acids as later discussed, for example, the velocity of the gas flow has to be adjusted to rates considerably exceeding the velocity of the liquid phase. The counter current flow of the gas quantities in opposite direction to the liquid decreases by friction forces the flow velocity of the liquid. As generally known, this might lead, particularly in the case of reactors having internals or built-in packings, to the generation of a self-generating flow barrier for the liquids in the reactor preventing disturbance-free operation of the reactor (the reactor xe2x80x9cfloodsxe2x80x9d). The limit of the respective gas or liquid velocity at which this phenomenon of flooding occurs, is also referred to as the flooding point. A stationary counter current process above the flooding point is not possible. Since it is necessary to operate below velocities at which flooding occurs, in many reactions, particularly in the case of ozonolysis, the possible reactor flow rates are too low.
In view of the above reasons, a reactor with co-current contact is frequently employed. In a co-current process with parallel flow, both the gaseous phase (carrier gas and gaseous reactive component) and the liquid phase flow in the same direction. In the case of stoichiometric use of the reactants and in view of the great density difference, a considerably larger volume flow for the gas as compared to the liquid, and hence a considerably higher flow velocity of the gas, has to be established. Friction forces cause an increase in the flow resistance at the interface between gas and liquid. Thereby it is possible to employ such built-in packings which increase the flow resistance thus reducing the flow velocity and enlarging the surface that would not be passed through by gas and liquids flow in a counter current process.
It is, however, a disadvantage of the co-current principle that considering the high flow velocity, the period of dwell of the gas and hence of the gaseous reactants in the reactor is relatively short. In co-current reactors, furthermore, an unfavorable distribution of temperature and concentration along the length of the reactor can be observed. When entering the reactor, both the liquid and the gas phase have high concentrations of the reactants. During the course of the process, however, the concentrations in both phases substantially decrease. In the case of exothermic reactions, for instance, this results in a high heat development, and thus a high material turnover at the entry of the components into the reactor, and in little heat development and thus in little turnover at the reactor exit. In order to nevertheless obtain a complete turnover, a plurality of co-current reactors are e.g. provided in cross flow arrangement to approximate the counter current principle.
In a counter current process, however, the turnover of the product is inherently uniformly distributed along the reactor length. By this kind of process technique, it is much easier to obtain complete turnover of the reactants. Likewise process control of exothermic reactions can be carried out in a considerably easier manner. One example of a gas-liquid multi-phase reaction known for a long time and carried out on an industrial scale where the above mentioned problems come up, is the above-referenced ozonolysis of unsaturated organic compounds with ozone in oxygen or air and subsequent oxidative cleavage of the ozonides generated in ozonolysis with oxygen or air. The oxidative cleavage of oleic acid by means of ozone and oxygen represents an industrially very significant application of this technique. Oleic acid is first reacted to the oleic acid ozonide in the presence of pelargonic acid and water with an ozone/oxygen mixture, or an ozone/air mixture. The reaction is carried out in a reaction column including, if need be, built-in packings. The resulting ozonide is subsequently cleaved, or further oxidized, respectively, in a second reactor to obtain the corresponding carboxylic acids.
Although the chemical principle of the reaction is rather simple and has been known for a long time, the technical implementation, for the reasons already mentioned, is rather sophisticated. It concerns a system wherein two immiscible liquids, water and organic acid, with oxygen as a gas, have to be reacted at substantial differences of the flow volumes of gas and liquids. The resulting oleic acid ozonide has substantially different physical and chemical properties (for instance regarding adhesive power, viscosity, solubility, polarity, melting point, emulsifying properties) compared to oleic acid. It is, inter alia, for this reason that in technical ozonolysis the oleic acid is added with a non-reacting second fatty acid, namely pelargonic acid. After the reaction of the oleic acid to ozonide, the pelargonic acid serves additionally as a solvent for the ozonide. In order to eliminate the reaction heat, water is added. The reaction is carried out so that the heat of the reaction is absorbed, on one hand, by the heat capacity of the reaction solution including water, and on the other hand primarily by evaporation energy which is removed from the reaction mixture by the evaporation of the liquid phase and particularly of the water of the through-flowing, originally dry, gas until steam saturation is reached.
The above mentioned cross current reactor arrangement comprising a plurality of co-current operated tube reactors was developed for ozonolysis of oleic acid. If ozonolysis is carried out in a reactor operating in accordance with the counter current principle, the latter is operated with small flow velocities of the phases and, in order to obtain any considerable material turnover, the reactor has, therefore, a correspondingly large volume, or large dimensions.
In addition to the above, gas-liquid reactions are also possible in bubble column reactors. In such reactors, a continuous liquid phase fills the major portion of the reactor volume in the form of a liquid column fed, for example, from the top, and a discontinuous gas phase in the form of bubbles is fed from the bottom through the aliquid column. In order to achieve a stoichiometric reaction, critical multi-phase reactions require relatively large gas volumes. The large gas volumes cause unacceptably long dwelling times of the liquid in the bubble column reactor. If, as is the case in the above ozonolysis of unsaturated fatty acids, the reaction product is explosive, this kind of reactor has to be ruled out alone for safety reasons.
Bubble column reactors, in addition, cause the problem that along the length of the reactor, the gas bubbles combine to form enlarged gas bubbles so that the reaction turnover decreases towards the reactor head. In order to solve this problem, it has been known from CH 507,734 A to provide a multi-stage bubble column by interrupting the bubble column by fine-pored gas barriers. The reactor interior then includes a plurality of bubble columns arranged one after the other. A gas inlet valve at the reactor bottom is synchronously or alternately switched on and off by means of a gas outlet valve at the reactor head. The same applies to the liquid valves at the upper and the lower ends of the reactor. The ascending gas bubbles collect and combine beneath the gas barriers to generate gas paddings which are pressed by periodically generated pressure impacts through the gas barriers above them in order to re-disperse the gas phase. The pressure impacts of the gas from below and of the liquid from above which act on the column contents are generated, in addition to a series of alternative measures, by alternatingly opening and closing the two control valves for the gas phase and the two control valves for the liquid phase.
It is an object of the present invention to provide a process allowing improved execution of multi-phase reactions according to the counter current principle also in the case of the above-mentioned critical phase combinations. Another object is to provide an apparatus for carrying out the process. The objects are solved by the subject matters of independent method and apparatus claim, respectively. Advantageous further developments are defined in subclaims.
The invention is directed to a process for carrying out a multi-phase reaction in a continuously operated counter current tube reactor in contrast to reactors operated in batch mode. Hence, modulations or short interruptions of input of gas and liquid phase into the reactor are possible in the invention.
Surprisingly, the inventors discovered a process for uninterruptedly operating a counter current tube reactor with a liquid phase running down as a flow film and a continuous gas phase flowing up at extraordinarily high phase velocities of the gas and liquid phase by pulsating the gas phase. In spite of the high velocities of the continuous, pulsating gas phase and of the continuously-fed liquid phase, stable reactor operation showing excellent reaction results and turnovers can be obtained in the reactor, whether with or without built-in packings. Corresponding results can otherwise be obtained only in parallel-flow reactors having for instance a much larger cross section and longer reaction times. In addition, by means of the pulsing of the continuous gas phase according to the present invention, the flooding point can obviously be raised.
The gas phase can be pulsated by repeated temporary lowering of pressure at the gas inlet to effectively counteract liquid phase stagnation and flow film thickness increase. Pulsating the gas phase can also be accomplished alternatively or additionally by a pressure increase at the gas discharge from the tube reactor.
In a reactor having no built-in packings in which high phase velocities cause stagnation points or flow barrier points, the inventive measure can avoid repeated increase of the flow film of the liquid phase on the tube wall up to a range of values unacceptable and even critical for the reaction (values at which, in extreme cases, collapsing or crashing down of the flow film within the reactor tube results). The novel gas phase pulsation induces a repeated increase and decrease of the flow film thickness and, thereby, an increase of the flow film surface and of the reaction face, occur along the length of the tube. The flow film surface takes a wave-like profile where the liquid phase in the flow film is advantageously shifted and displaced in itself and thereby different liquid phase areas in the film profile get into contact with the gas phase. Advantageously, the repeated increase of the gas phase pressure prevents a decrease of the flow film to too low values.
By the temporary pressure decrease in the gas phase from below (and/or pressure increase from above, respectively), the counter flow movement in the gas phase is temporarily interrupted so that the gas phase comes at least to a standstill. In accordance with the invention, however, it is preferred that the flow direction of the gas phase is temporarily reversed. It is surprising that by doing so the stable, continuous operation of the reactor is not disturbed, rather the reaction process is evidently promoted. One reason may be that the gas phase by means of its stroke movement may come into reaction with the liquid phase over longer contact times and increased phase exchange faces. Such positive effect of the increase of the reaction turnover, however, cannot be obtained by merely switching on and off the gas supply through a valve.
Moreover, a further increase of the turnover of the reaction can be obtained by not only decreasing the gas phase pressure in a repeated manner for a certain period of time but rather periodically increasing and decreasing the gas phase such that the pulsed gas pressure causes an oscillation or vibration of the gas phase in the tube reactor. The generated gas oscillation induces a periodic movement of the gas phase opposite to the given counter current direction and promotes the reaction process.
The liquid phase is also advantageously intermixed by turbulence generated by the repeated, preferably periodic, gas stroke. This leads to an increase of the material turnover potential per reaction volume. In addition, even in the case of extremely fine-meshed structures, areas clogged by the liquid are cleared by the gas pulsation. Therefore, in accordance with the invention, tightly packed fine-meshed structures can be provided in the counter current reactor.
A particular advantage of the present invention consists in that operation is possible at a very high gas flow velocity, which is of decisive advantage for chemical reactions requiring stoichiometric ratios, such as the two-step oxidative ozonolysis of unsaturated fatty acids. In view of the improved turnover rate, operation at high volume flow rates and very high ozone concentrations can be achieved. The water addition can be increased within large limits without any narrow restriction by the other components of the liquid phase.
In accordance with a preferred embodiment of the present invention, liquid phase stagnations are avoided. In any case, the pressure of the gas phase will always be temporarily changed and pulsed at least, whenever liquid phase stagnation or unacceptable increase of flow film thickness occur. The temporary pressure change can be adjusted to provide a pressure decrease below the location of stagnation and decrease the gas phase velocity to allow the stagnating liquid phase to dissipate and reduce the flow film thickness to a desired value. In this way, the reactor can have, if desired, local stagnations for short periods of time while the operation thereof does not become discontinuous. In other words, the well-known phenomenon of flooding in packed gas-liquid reactors with continuous gas phase and a free-falling liquid film can be overcome by the present invention. The flow characteristics of the two phases can approximately be considered as plug flows.
The periodic vibrations of the gas conducted in the counter flow induce, also in the liquid phase and vis-a-vis the gas itself, a substantially better mixing than in the case of a counter current process without periodic vibration. The introduction of periodic vibration has already been employed in liquid-liquid counter current percolators. The two phases in these percolators, however, vibrate inevitably in-phase. This does not hold true in gas-liquid multi-phase reactors, and the flow situations are not comparable.